1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
There have been utilized image forming apparatuses that control fluctuation in speed of an image carrier and an intermediate transfer member. For example, Japanese Patent Application Laid-Open No. H6-274051 discloses a conventional image forming apparatus in which, when an image is not formed on an image carrier, at least one of the image carrier and a pressure roll is moved to separate press-contact portions therebetween with a driving force from driving means for driving to rotate the image carrier and, in synchronization with timing when a recording medium enters between the press-contact portions of the image carrier and the pressure roll, releases the separation of the press-contact portions using an urging force of urging means for bringing the image carrier and the pressure roll into press contact with each other.
Japanese Patent Application Laid-Open No. H4-242276 discloses another conventional image forming apparatus that pushes down a transfer member by a push-down amount corresponding to the thickness of a recording medium and forms a gap between an image carrier and the transfer member to control fluctuation in speed of the image carrier that occurs when the recording medium enters a press-contact portion of the image carrier and the transfer member.
Japanese Utility Model Publication No. S56-47639 discloses yet another conventional image forming apparatus in which a concave groove is provided in an image carrier or a transfer member and a gap narrower than the thickness of a recording medium is formed in a press-contact portion of the image carrier and the transfer member to control fluctuation in speed of the image carrier that occurs when the recording medium enters the press-contact portion of the image carrier and the transfer member.
Japanese Patent Application Laid-Open No. S61-90167 discloses yet another conventional image forming apparatus in which a step as a shock absorbing material is provided on a circumference of a roller of a transfer roller or a pressure roller to relax a shock.
In a press-contact portion of a transfer unit of a conventional image forming apparatus, as shown in FIGS. 67 to 70, a sheet 58 is conveyed to the press-contact portion of a counter roller 56 and a transfer roller 57 in synchronization with a leading end of a toner image on an intermediate transfer belt 52. The toner image born on the intermediate transfer belt 52 is transferred onto the sheet 58 with a pressing force by a compression spring 59 and a transfer bias (not shown). A state at the time when a leading end of the sheet 58 enters the press-contact portion of the transfer unit is shown in FIGS. 67 and 68. A state at the time when a trailing end of the sheet 58 exits the press-contact portion of the transfer unit is shown in FIGS. 69 and 70. Sectional views of the transfer unit in a width direction of the sheet 58 are shown in FIGS. 67 (a), 68 (a), 69 (a), and 70 (a). Sectional views of the transfer unit in a conveying direction of the sheet 58 are shown in FIGS. 67 (b), 68 (b), 69 (b), and 70 (b). When the leading end of the sheet 58 enters the press-contact portion of the transfer unit, as shown in FIG. 67 (b), the transfer roller 57 is pressed against the intermediate transfer belt 52 by the compression spring 59 and the leading end of the sheet 58 is caught between the intermediate transfer belt 52 and the transfer roller 57 and enters the press-contact portion. As shown in FIG. 68 (b), the transfer roller 57 is pushed down by an amount equivalent to the thickness of the sheet 58. When the trailing end of the sheet 58 exits the press-contact portion of the transfer unit, as shown in FIG. 70 (b), the transfer roller 57 pushed down as shown in FIG. 69 (b) is pushed up by a pressing force of the compression spring 59 by an amount equivalent to the thickness of the sheet 58 to come into a state after the passage of the sheet 58.
In a conventional image forming apparatus, as shown in FIG. 71, toner images are formed by image forming units 201Y, 201M, 201C, and 201BK corresponding to four colors of yellow, magenta, cyan, and black, respectively, primarily transferred onto an intermediate transfer belt 204, and carried by the intermediate transfer belt 204. The intermediate transfer belt 204 is stretched and suspended by a belt driven roller 208, a counter roller 209, a belt stretching and suspending roller 210, and the like. A belt driving roller 207 is driven by a belt driving motor 205 and a belt driving gear 206. A recording sheet 228 is conveyed by a sheet separating mechanism (not shown) and conveying means and registered by a registration roller pair 212 at desired timing and passes a recording sheet conveying path 213. A leading end of the recording sheet 228 passes a sheet registration sensor 221. The recording sheet 228 is conveyed to a secondary transfer nip section where the intermediate transfer belt 204 is nipped by the counter roller 209 and a transfer roller 215. The recording sheet 228 is heated and fixed by a fixing roller pair 214 and output.
However, in the image forming apparatus described in Japanese Patent Application Laid-Open No. H6-274051, since the press-contact portions are separated until a recording medium enters between the press-contact portions, it is possible to expect that a rotation load of the image carrier is controlled when the recording medium enters between the press-contact portions. However, when the separation of the press-contact portions is released, since the image carrier, the recording medium, and the transfer roller collide with one another because of the urging force of the urging means, a rotation load is generated in the image carrier to cause deterioration in an image. Specifically, as shown in FIG. 64, when a gap between a transfer roller 503 and an image carrier 501 is released, since the transfer roller 503 collides with the image carrier 501 because of an urging force of a spring 504 coupled to an arm 502, a rotation load is generated.
In the image forming apparatus described in Japanese Patent Application Laid-Open No. H4-242276, the transfer member is pushed down by a push-down amount corresponding to the thickness of the recording medium and the gap is formed between the image carrier and the transfer member to obtain, regardless of the thickness of the recording medium, an effect of reduction in fluctuation of speed at the time when the recording medium enters the press-contact portion. However, since a device for detecting the thickness of the recording medium has to be provided and an amount of adjustment has to be determined according to the thickness of the recording medium to drive the transfer member, the image forming apparatus is expensive and complicated. As shown in FIG. 65, since it is necessary to provide a motor 511 separately from a driving circuit for rollers to form the gap, cost is further increased.
In the image forming apparatus described in Japanese Utility Model Publication No. S56-47639, since the gap is formed using the concave groove, it is possible to expect that a rotation load of the image carrier at the time when a recording medium enters the press-contact portion is controlled. However, since the image carrier or the transfer member comes into contact with only a portion where the concave groove is provided, concentration of stress locally occurs only in that portion. Thus, it is likely that plastic deformation occurs in a long term use. Specifically, as shown in FIG. 66, since an end of a roller 521 is always in press contact with a roller 522, local concentration of stress occurs in the press-contact portion. Thus, it is likely that the rollers are deformed in a long term use.
In the image forming apparatus described in Japanese Patent Application Laid-Open No. S61-90167, since a gap is formed in a fixed size, a conveying force is insufficient when a recording sheet is thin. Thus, a slip phenomenon in which the recording sheet cannot be conveyed to a downstream side occurs. Further, deficiencies such as a skew phenomenon (skew feeding) occurs because of imbalance of frictional forces at both ends of the rollers.
In a conventional electrophotographic image forming apparatus, a transfer system may be employed for transferring an image onto a recording sheet by pressing a transfer roller against an image carrier while the recording sheet is nipped between the transfer roller and the image carrier. In general, in the image forming apparatus, a phenomenon in which conveying speed of the image carrier temporarily falls when a leading end of the recording sheet enters a press-contact portion of the transfer roller and the image carrier occurs. In particular, the phenomenon is more conspicuous as a recording sheet is thicker. In recent years, since a distance from a registration roller to the press-contact portion of the transfer roller and the image carrier is reduced according to the demand for reduction in size of apparatuses, fluctuation in speed of the image carrier imposes a problem more serious than in the past.
Because of this phenomenon, linear speed of not only a transfer process but also processes of cleaning, exposure, and development performed on a photosensitive member fluctuates. Thus, image failures called banding such as streaks in a main scanning direction, band-like density unevenness in a sub-scanning direction, and image deviation occur.
As shown in FIGS. 67 to 70, in the conventional image forming apparatus, when the leading end of the sheet 58 enters the press-contact portion of the transfer unit, a rotation torque of the counter roller 56 is used via the intermediate transfer belt 52 to push down the transfer roller 57 by an amount equivalent to the thickness of the sheet 58. Thus, a rotation load of the counter roller 56 and a traveling load of the intermediate transfer belt 52 are generated. Therefore, positional deviation occurs in a primary transfer unit between a photosensitive drum and the intermediate transfer belt 52 and image density unevenness occurs in an image formed on the sheet 58. When a distance between the two transfer units, i.e., a distance from the primary transfer unit to the secondary transfer unit is L1, the density unevenness in this case occurs between the leading end of the sheet 58 and a position of L1 from the leading end. On the other hand, when the trailing end of the sheet 58 exits the press-contact portion of the transfer unit, a load torque is generated in a rotating direction of the counter roller 56 via the intermediate transfer belt 52. Thus, the rotation of the counter roller 56 is accelerated and the traveling of the intermediate transfer belt 52 is also accelerated. Therefore, positional deviation occurs in the primary transfer unit and image density unevenness occurs in the image formed on the sheet 58. When a distance from the trailing end of the sheet 58 to a leading end of the next sheet is L2 and the length in the sub-scanning direction of the sheet 58 is L, the density unevenness in this case occurs in a position of L1-L2 from the leading end of the next sheet 58 if L2<L1<L2. The problems described above occur not only in the press-contact portion of the transfer unit but also in the press-contact portion of a registration unit located upstream in the sheet conveying direction of the transfer unit.
As shown in FIG. 71, in the conventional image forming apparatus, transfer roller shafts 215a at both ends of the transfer roller 215 are supported by bearings 220 and outer peripheries of the bearings 220 are supported by slide bearing holders 219. The outer sides of the slide bearing holders 219 slides in slide holes 217 opened in a main body side plate 216. Thus, the transfer roller 215 has a degree of freedom in a normal direction that is in contact with the counter roller 209. Moreover, press springs 218 including compression springs are provided between the slide bearing holders 219 and spring receiving sections 216a. Thus, when the leading end of the recording sheet 228 reaches the nip section, the transfer roller 215 slides to a lower side in the figure according to the thickness of the recording sheet 228. When the trailing end of the recording sheet 228 exits the nip section, the transfer roller 215 slides to an upper side in the figure according to the thickness of the recording sheet 228. Thus, when the recording sheet 228 is thick, the transfer roller 215 sharply moves up and down when the leading end and the trailing end of the recording sheet 228 pass the nip section.